[HN Gopher] How far could the sun possibly be?
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How far could the sun possibly be?
Author : cwillu
Score : 103 points
Date : 2024-01-21 23:52 UTC (23 hours ago)
(HTM) web link (profmattstrassler.com)
(TXT) w3m dump (profmattstrassler.com)
| cwillu wrote:
| Series starts at https://profmattstrassler.com/2024/01/16/the-
| value-of-check-...
| ltbarcly3 wrote:
| This is a great introduction into how real astrophysics is done.
| We don't know how far away things are until we measure, but our
| measurements are never precise, and we don't have very many
| 'calibrated' distances to compare to. Little by little it's
| possible to exclude possibilities and narrow down on the likely
| true value.
| ForOldHack wrote:
| I almost got my degree on astrophysics. Yes, I read a lot of
| bad and good textbooks. This one is rather brilliant, lucid,
| and well done.
|
| But I have to follow up with: "Well, but... how do we really
| know?" A: We have followed up well worn paths of scientific
| inquiry, and looked at our assumptions, in designing an
| experiment that, we assume to fail. When it does not follow our
| assumption, then we can only rule out failure, and call it a
| success. This follows centuries of this type of exacting and
| painstaking scientific work. Although I would normally have 10
| ~ 12 citations of very diverse work, that exercise is left to
| the reader.
| empath-nirvana wrote:
| One thing he sort of implies but doesn't directly state -- which
| I think a lot of people don't know -- is that it's impossible to
| measure the "one way" speed of light. We can only measure the
| speed it takes for light to go "there and back". It's possible
| (but not likely), that light goes faster in one direction than in
| another direction, and AFAIK, there's no possible way to measure
| it. You'd think you could do it based on clock synchronization,
| but clock synchronization itself depends on the assumption that
| the speed of light is equal both directions.
|
| https://en.wikipedia.org/wiki/One-way_speed_of_light
| readams wrote:
| Also a Veritasium video on the subject:
| https://www.youtube.com/watch?v=pTn6Ewhb27k
| andersa wrote:
| Why doesn't it work to have the emitter and sensor together at
| the same location, synchronize them at that moment, and then
| move them apart a distance so large the initial delay no longer
| matters, before running the test? Do we not have accurate
| enough clock sources to keep the synchronization?
| Tagbert wrote:
| Moving the emitters affects the pace of time for them.
| AnimalMuppet wrote:
| Right, so move them at identical speeds, with identical
| acceleration profiles.
| rjp0008 wrote:
| If you do this, the clocks will be in the same place, it
| would have to be opposite acceleration profiles to get
| them moving away from each other.
| JumpCrisscross wrote:
| > _move them at identical speeds, with identical
| acceleration profiles_
|
| Now do GR.
| AnimalMuppet wrote:
| That's the reason for the identical acceleration
| profiles.
|
| Which is totally obvious, so I suspect that means that I
| missed your point. Could you clarify?
| JumpCrisscross wrote:
| Mass and energy curve spacetime. So you could accelerate
| two clocks identically and still have to correct for
| nearby mass and energy.
| AnimalMuppet wrote:
| Well, sure. I was thinking of stations that are a few
| tens of km apart on a flat region of Earth, so I don't
| think that would be much of an issue.
| adgjlsfhk1 wrote:
| the assumption that time dilation is identical for the
| same acceleration profiles is equivalent to an assumption
| of the 1 way speed of light. if you do the full math with
| a non constant light speed, you find that degree of
| asymmetry in the 1 way speed of light directly cancels
| the difference in time dilation
| AnimalMuppet wrote:
| Well, how about this: I have a central facility. In that, I
| synchronize several clocks. I then slowly move them to
| satellite facilities in opposite directions. I don't move all
| the clocks by the same path - some go via triangular routes
| rather than directly.
|
| If all the clocks agree at the satellite facilities, then I
| have established that space is isotropic for the slow transport
| of clocks (or at least, it is isotropic for the paths chosen -
| a skeptic can always device a "sufficiently smart anisotropy"
| that would appear to be isotropic for the paths chosen). Per
| the article, that was one of the assumptions that couldn't be
| trusted, but if we can experimentally establish it, we can
| trust it.
|
| We now have synchronized clocks at the two satellite
| facilities. (We know they're synchronized because we
| established that space is anisotropic to the slow transport of
| clocks, and also because at least some of the clocks were
| transported with identical profiles in opposite directions.) We
| can now use time of receipt minus time of transmit to establish
| the one-way speed of light.
| mecsred wrote:
| How do you measure if the clocks agree or not after you move
| them? You can try and synchronize all the moved ones at point
| B, but how do you measure their relative timing to A clocks
| without relying on the speed of light between A and B.
| floxy wrote:
| You could bring clocks A & B back together again.
| dilyevsky wrote:
| And by doing this you reversed direction and didn't
| actually measure "one way"
| floxy wrote:
| Synchronize A & B in one location. Move A & B apart in a
| careful manner. Send light pulse from A and record time-
| stamp on A's clock when pulse sent. When pulse is
| received at B, record time stamp on B's clock. Return
| clocks A & B together (in a careful manner) to confirm
| they are still in sync. Compare time stamp between A's
| transmission, and B's reception. Who knows, maybe when
| you bring them together, clocks A and B aren't in sync,
| due to some twin-paradox thing. Maybe you can't be
| careful enough.
| dilyevsky wrote:
| > Synchronize A & B in one location. Move A & B apart in
| a careful manner.
|
| Doesn't matter how careful you are, SR tells us moving
| clock will become unsynched. The amount of "unsynching"
| depends on c (see Lorentz factor) so if c is different in
| forward vs reverse direction, bringing the clocks back
| will even it out
| burkaman wrote:
| > If all the clocks agree at the satellite facilities
|
| The idea is that you can't know this. You're somewhere in the
| middle receiving messages from all the clocks, and you can't
| tell if they're synchronized unless you've already defined
| the speed of light between you and each clock.
| foobarian wrote:
| I suppose you could synchronize them first, then swap their
| positions randomly, and try again.
| AnimalMuppet wrote:
| Perhaps I said that badly. "If all the clocks agree at
| satellite facility A, and all the clocks agree at satellite
| facility B". I'm not comparing clocks at A with clocks at
| B.
|
| Second, I was thinking of labs perhaps tens of km apart.
| You can have people at the center, and at satellite
| facility A, and at satellite facility B.
| vikingerik wrote:
| Here's the problem: you can only observe a remote clock, or
| any remote object that light from your source reached, _at
| the speed of the light that traveled back from it_.
|
| Put another way: the speed of any signal or causality _coming
| back from your measuring device_ is always a factor with no
| way around that.
| floxy wrote:
| He is just logging timestamps of when the signal arrived.
| It shouldn't matter if the timestamp gets back to the
| central location by carrier pigeon. But maybe the catch is
| that it doesn't matter how slowly he moves the clocks into
| position, they'll always be skewed by time dilation.
| AnimalMuppet wrote:
| > But maybe the catch is that it doesn't matter how
| slowly he moves the clocks into position, they'll always
| be skewed by time dilation.
|
| I dealt with that by moving the clocks with identical
| velocity profiles, so time dilation should be the same...
|
| Unless time dilation is anisotropic. I dealt with _that_
| by sending multiple clocks, with some sent on triangular
| routes and some direct. In more detail:
| C A B D
|
| If I send a clock from A to C to B, and a clock from A to
| D to B, and the two clocks arrive with the same time,
| then I have evidence that time dilation is anisotropic
| (for at least those two routes). I _don 't_ necessarily
| expect that they have the same time as a clock sent
| direct from A to B - they have an additional
| acceleration, from the change of direction at C or D, and
| they have more time at velocity, because of traveling the
| longer distance. I think I said that very badly in my
| first post.
|
| But the point is, if I can show that time dilation is
| anisotropic, then the clocks that went direct from A to
| B, and the clocks that went the same distance in exactly
| the opposite direction, should have the same time on
| them.
| lmm wrote:
| > If I send a clock from A to C to B, and a clock from A
| to D to B, and the two clocks arrive with the same time,
| then I have evidence that time dilation is anisotropic
| (for at least those two routes).
|
| You mean isotropic, and you don't really. D->B is the
| same as A->C and C->B is the same as A->D; whatever
| clever path you come up with, a clock going from A to B
| will end up having had vertical movements that sum up to
| 0. If moving up induces some extra time dilation and
| moving down reduces it, or vice versa, you'll never be
| able to detect it; ultimately you can only ever make
| measurements when you and your clocks (and/or signals)
| have moved in closed loops, however squiggly.
| AnimalMuppet wrote:
| OK, so that diagram was intended to be horizontal, not
| vertical. Obviously you want to minimize vertical
| movement as much as possible.
|
| But I see what you mean about the sides (as drawn) being
| parallel.
|
| And, yes, I meant isotropic, not anisotropic.
| Embarrassing.
|
| OK, how about this: I have an equilateral triangle, with
| vertices A, B, and C. I synchronize clocks four clocks at
| A. I send one clock to B directly, and one to C and then
| B. I send one clock to C directly, and one to B and then
| C. I do the same from points B and C. Then, I can look at
| the difference between clocks that came direct and clocks
| that came the long way. If all the differences are the
| same, then I can say that going A-to-B-to-C has the same
| effect as going A-to-C-to-B or B-to-A-to-C or any other
| route. Doesn't that show isotropy?
| seiferteric wrote:
| Is it possible to use redshift of photons from the sun? Say if
| you know the hydrogen transition line frequency, then measure
| very precisely the observed frequency from solar photons, you
| could calculate the redshift. I suppose this would rely on
| knowing the mass of the sun already as well.
| Someone wrote:
| I think redshift correlates with relative velocity and
| expansion of the universe, not with distance.
|
| https://en.wikipedia.org/wiki/Redshift:
|
| _"The main causes of electromagnetic redshift in astronomy
| and cosmology are the relative motions of radiation sources,
| which give rise to the relativistic Doppler effect, and
| gravitational potentials, which gravitationally redshift
| escaping radiation. All sufficiently distant light sources
| show cosmological redshift corresponding to recession speeds
| proportional to their distances from Earth, a fact known as
| Hubble 's law that implies the universe is expanding."_
| seiferteric wrote:
| I thought so as well but recently discovered:
| https://en.wikipedia.org/wiki/Gravitational_redshift
|
| "gravitational redshift (known as Einstein shift in older
| literature)[1][2] is the phenomenon that electromagnetic
| waves or photons travelling out of a gravitational well
| (seem to) lose energy. This loss of energy corresponds to a
| decrease in the wave frequency and increase in the
| wavelength, known more generally as a redshift. "
| gizmo686 wrote:
| Isn't this essentially essentially the ether theory of light?
| You can measure the round trip of light along two different
| axis. If the speed of light was dependent on direction, you
| would expect these results to differ.
|
| It is possible that physics conspires such that the speed of
| light is direction dependent, but that it averages out if half
| your path is the exact opposite direction from the other. I
| think this can be excluded by comparing more complicated paths;
| although the nessesity for it to form a closed loop might be
| give physics an unavoidable out if it really wanted to mess
| with us.
|
| There are also theories where the speed of light differs based
| on direction; but space itself differs in the same way,
| canceling the effect. These are fundamentally equivelent to a
| theory where both are constant.
| ForOldHack wrote:
| No, it is not the ether theory of light, and that is a
| magnificent question: This idea was settled by
| Mickelson/Morley, who not only measured the round trip in one
| direction, but measured it in several directions, and found
| the speed of light to be invariant. For which Albert Einstein
| received the Nobel Prize for his theory of light.
|
| Later, Richard Feynman used first principals to both confirm
| this for Enistienien physics, and break it for quantum
| physics.
|
| The best reference for this work is not the classic
| experiment, but on Henry Cavindish's balance, which led to
| the calculation of G, the gravitation constant to 7 digits of
| accuracy, based upon the speed of light calculated to 9+
| digits of accuracy.
|
| The speed of light is invariant: What you the observer
| actually see, is a frame of reference in space-time, which
| transforms the space, so that light still travels as fast as
| it always does, but the space around it is transformed.
|
| There have been a few theories of exceptional note: Sir Fred
| Hoyle solved Einsteins equations for an invariant size of the
| universe based on a shrinking frame of reference, and found
| no contradictions. Hence the wimper theory of cosmogony. I
| count myself as pretty bright, on this subject, able to argue
| the point rather succinctly, but I never claim to hold a
| candle and a mirror ( Cavendish ) to Henry Cavendish, nor Sir
| Fredric Hoyle: You want to get the real brilliance of this
| total failure:
|
| "The Michelson-Morley Experiment (MMX) tried to prove the
| existence of ether, but they did not observe the movement of
| interference fringes, which led to the assumption that the
| speed of light is constant in the inertial reference frame,
| which is also the theoretical basis of Einstein's special
| relativity (SR)."
|
| It failed to prove the existence of ether. Failed. Richard
| Feynman also found that for Eisensteinian physics, this was
| also true from first principals. This is really one of the
| most brilliant failures in the history of Physics.
|
| "Success is the ability to go from failure to failure without
| losing your enthusiasm" -- Winston Churchill
| dilyevsky wrote:
| Not the same as MM test. The one-way is principally
| unknowable because our fastest way to transmit information is
| limited by the speed of light itself
| whatshisface wrote:
| It's impossible to measure because it has no real existence.
| The one-way speed of light is as metaphysical of a quantity as
| the British pound. Numerical speeds can be whatever you want
| them to be in an arbitrarily curved coordinate system - and the
| speed of light is defined in the "flattest" one of them.
| MiguelX413 wrote:
| The British pound is 0.45359237 kg, no?
| whatshisface wrote:
| It could have been 0.5639472942kg just as easily.
| gumby wrote:
| I think we need an investigation into why the British
| Pound isn't allowed to float.
|
| The fixed exchange rate between pounds and kg is obsolete
| and inappropriate for a modern economy and is the kind of
| thing BREXIT was supposed to free us from.
| bee_rider wrote:
| I'm pretty sure a pound could float, as long as it was
| made large enough.
| kbenson wrote:
| Or 1.27 USD. Today. According to Google. Which is pegged to
| some market rate at some time. Which may differ based on
| your exact location and the conditions in the area. And is
| expected to be different a short time from now.
| edgyquant wrote:
| What exactly is a kg?
| MOARDONGZPLZ wrote:
| Basically 1,000 grams.
| edgyquant wrote:
| What is a gram?
| ForOldHack wrote:
| A gram is the division of a standard Kilogram (Kg ) into
| 1000 divisions. It's a poor description to both be
| circular about it, but the Kg is the standard measure of
| mass. Look to The definition of the standard kilogram.
|
| "Since the revision of the SI on 20 May 2019, we can now
| compare the gravitational force on an object with an
| electromagnetic force using a Kibble balance. This allows
| the kilogram to be defined in term of a fixed numerical
| value of the Planck constant, a constant which will not
| change over time."
|
| "A Kibble balance is an electromechanical measuring
| instrument that measures the weight of a test object very
| precisely by the electric current and voltage needed to
| produce a compensating force. It is a metrological
| instrument that can realize the definition of the
| kilogram unit of mass based on fundamental constants."
|
| "One important reason for the change is that Big K is not
| constant. It has lost around 50 micrograms (about the
| mass of an eyelash) since it was created. But,
| frustratingly, when Big K loses mass, it's still exactly
| one kilogram, per the current definition. When Big K
| changes, everything else has to adjust."
| kibwen wrote:
| Basically 1 gram.
| bbojan wrote:
| 1/12th of the weight of 6.02214076x10^20 carbon 12 atoms.
| ForOldHack wrote:
| Speed has no existance in space, it requires the passage of
| time. The one way speed of light can be measured, and is very
| important to the design of curcits. Anytime you wish to argue
| this point, take it up with RtAdrm Grace Hopper. I count
| myself as very very bright, but I do not hold a burning punch
| card to Hopper.
|
| "Since 1 July 1959, the international avoirdupois pound
| (symbol lb) has been defined as exactly 0.45359237 kg. In the
| United Kingdom, the use of the international pound was
| implemented in the Weights and Measures Act 1963. (a) the
| yard shall be 0.9144 metre exactly; (b) the pound shall be
| 0.45359237 kilogram exactly."
|
| "The kilogram is defined by taking the fixed numerical value
| of the Planck constant, , to be 6.626 070 15 x 10-34 when
| expressed in the unit J s, which is equal to kg m2 s-1, where
| the metre and the second are defined in terms of the speed of
| light, , and the hyperfine transition frequency of the
| caesium-133 atom..."
|
| Nothing new. Rando hacker news poster, vs say... the
| International Standards Organization on Weights and measures.
| Hmm... I am going to place my faith in say... a group of
| people who's degrees far outnumber most of the colleges I
| studied at.
|
| No, the speed of light is not defied in the "flattest" one of
| them. Please do your homework. "The speed of light is a
| universal constant denoted by c."
|
| In my second college physics class, the final exam was one
| single question: "Derive the speed of light." I got a grade
| of 4/10, which put me at the top two students in the class.
| The class was a 5 1/2 month exercise in brutality of math. I
| would suggest you get a few college physics classes under
| your belt.
| dilyevsky wrote:
| > The one way speed of light can be measured
|
| Incorrect. It is two-way you're measuring. Always. Yes,
| including in circuit designs
| TheOtherHobbes wrote:
| I suspect you may be missing one or two subtleties in this
| discussion.
| x3n0ph3n3 wrote:
| If the speed of light were different in one direction, the CMB
| would not look as uniform as it is.
| ars wrote:
| Only in a closed universe, where light "wraps around".
| Otherwise it would look exactly the same.
| x3n0ph3n3 wrote:
| That doesn't sound right. Please explain.
| ivalm wrote:
| To be fair, there is Doppler shift in cmb, we are moving
| about 370km/s relative to cmb rest frame.
|
| You can possibly imagine a world where some of this asymmetry
| is from a lorentzian ether.
| x3n0ph3n3 wrote:
| That's fair, but _highly_ improbable!
| cwillu wrote:
| I can't find the link offhand, but he's discussed this on
| another page, or possibly in one of his videos (which would
| explain why I couldn't find it in 30 seconds).
| LordGrey wrote:
| Even light, which travels so fast it takes most races thousands
| of years to realize that it travels at all, takes time to
| journey between the stars.
|
| -- Douglas Adams
| ivalm wrote:
| I think it doesn't matter, since Lorentz shrinking of ether in
| non-symmetric speed of light would shrink the faster direction.
| Civitello wrote:
| So, without reading, here are my uninformed thoughts on how I
| would do it:
|
| You could probably figure out the distance by using a technique
| we use to figure out the distance to nearby stars, measuring the
| change in position in the sky relative to very far away stars. I
| think you'd only need to observe two stars to figure out the
| distance.
|
| Could also use pulsar timing like gps signals to track the
| location of the Earth throughout its orbit.
|
| Could also take measurements after launching a pair of space
| probes away from the earth.
| mannykannot wrote:
| Unfortunately, the parallax method depends on knowing the
| diameter of Earth's orbit, which is (except for a factor of
| two) the value we are seeking. There's also the issue that the
| change the Sun's position with respect to the stars over six
| months is about 180 degrees (with some fluctuation, perhaps,
| depending on where the earth is in relation to its orbit's
| major axis when the measurement begins), and will be regardless
| of the Sun's distance.
|
| The distance of Venus was measured by the parallax method
| during a transit, with a baseline on the Earth's surface. This
| then yields all the other planets' distances from their orbital
| periods. This has me wondering why this had not been done for
| the Sun's distance, and perhaps the first reason to be
| considered is the difficulty of observing the Sun eclipsing
| distant stars.
|
| Update: According to Wikipedia [1], Jeremiah Horrocks came up
| with reasonable figures for both the size of Venus and the
| distance of the Earth from the Sun from a single observation of
| a transit, but the article says he made use of a false premise,
| so does that just mean he was lucky?
|
| [1]
| https://en.wikipedia.org/wiki/Transit_of_Venus#1639_%E2%80%9...
| ginko wrote:
| There was a pretty good CCC talk on this a couple years ago:
|
| https://www.youtube.com/watch?v=HFWV6XAXyx0
| g4zj wrote:
| This is way outside my area of expertise (if I have one at all),
| but assuming we know the diameters of the sun, Earth, and our
| moon, as well as the distance between Earth and our moon, could
| we not determine the distance between Earth and the sun based on
| the scale of the shadow Earth projects onto the moon during a
| partial lunar eclipse?
| gpm wrote:
| If you know the diameter of the sun you should be able to
| measure the distance by just measuring the size of the sun in
| the sky and using some simple geometry. I.e. if the sun takes
| up x degrees of space in the sky then tan(x) = (radius of sun /
| distance to sun).
|
| But... how do you measure the diameter of the sun?
| andrewclunn wrote:
| If you sent out probes to calculate the travel time of beams
| of light between two points, but at Lagrange points from one
| another, and then had them simultaneously slow their orbits
| to fall into the sun, and considered the "edge" of the sun
| the point at which it destroyed the probes...
| lifeisstillgood wrote:
| How do you measure the diameter of the sun when your most
| advanced technology does not include a telescope. And even
| when it does, but just a telescope about 8x power (approx
| Gallileo's first one)
|
| That Copernicus got there at all is incredible
| pdonis wrote:
| The problem is that the only way we have of knowing the Sun's
| diameter is to know its distance and then calculate its
| diameter from its apparent size.
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